Shortly before his 30th birthday, he became a junior minister in the Cabinet Office in 2001, following the recent election. In 2002, he was appointed Parliamentary Under-Secretary of State to the Office of the Deputy Prime Minister. He then moved to spend almost two years as Parliamentary Under-Secretary of State in the Department for Constitutional Affairs, working again under Falconer from 2003 to 2005.[1] He never rebelled against a Government position during his first time in Parliament[3] including voting in favour of the invasion of Iraq in March 2003.[5]

In September 2011, he stood in the shadow cabinet elections but missed out on becoming a shadow cabinet minister, however he was promoted to Her Majesty's Opposition becoming Shadow Financial Secretary to the Treasury. He replaced Stephen Timms, who was made Shadow Minister of State for Employment. On 7 October 2013, he was promoted to the Shadow Cabinet, becoming Shadow Chief Secretary to the Treasury. In May 2015, he was promoted to Shadow Chancellor, replacing Ed Balls, who had lost his parliamentary seat in the 2015 general election.

In June 2018 Leslie published a pamphet through the Social Market Foundation, where he is a member of the Policy Advisory Board,[13] entitled Centre Ground: Six Values of Mainstream Britain.[14] In August the same year The Guardian reported that "many saw the document as laying the intellectual groundwork for a future new [political] party,"[15] however Leslie denied this.[16]

In September 2018, Leslie lost confidence vote by his CLP and became the fourth Labour MP to have a motion passed against him. The motion, brought by members of the Mapperley branch of Nottingham East, attacked Leslie for his “disloyalty and deceit”, which it dubbed “a severe impediment to Labour Party electability”, and “incompatible” with Leslie continuing as the Labour candidate for Nottingham East. [17] Leslie did not attend the vote and had earlier remarked that the party had been infiltrated by the "intolerant hard left".[18] Centrist Labour MPs rallied around Leslie online.[19]

In February 2005, he married Nicola Murphy, a special adviser to Gordon Brown, in Westminster;[20] the couple became engaged the previous year.[21] In April 2016, Nicola Murphy founded Labour Tomorrow, an organisation which funds Labour-connected activists and groups who oppose Jeremy Corbyn as party leader.[22][23]